The experimental evidences and available theoretical explanations on formation of
stable clusters in colloidal suspensions are reviewed. The clusters form in the
parameters range intermediate between that corresponds to a stable suspension built
up by singlets and that causing the irreversible coagulation of the suspension. The
stable clusters can appear as a result of a competition between a short range attraction
and a long range repulsion between colloidal particles or due to reversible flocculation
in the shallow secondary potential well.